Fluoropolymer adhesion

ABSTRACT

A process for the improvement of adhesion between fluoropolymers or fluorocopolymers and inorganic compounds as well as composites made by this process are disclosed. The inorganic compound is coated with an adhesion promoter. Contacting the adhesion-promoted inorganic compound with the fluoropolymer or fluorcopolymer and heating develops the adhesive bond.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional application60/000,882, filed Jul. 12, 1995 and PCT International ApplicationPCT/US96/11491, filed Jul. 10, 1996, wherein the United States was adesignated country.

FIELD OF THE INVENTION

This invention concerns a process for the improvement of adhesionbetween fluoropolymers or fluorocopolymers and inorganic compounds andto composite materials made by this process.

TECHNICAL BACKGROUND

Various approaches have been used in the past in attempts to increasethe adhesion between fluoropolymers and substrates or filler materials.

U.S. Pat. No. 3,787,281, Effenberger, discloses a process for bondingpolytetrafluoroethylene to glass, fibers or plates, using a hydrolyzablesilane or a methacrylato chromic chloride as bonding agent.

Canadian Patent No. 764,537 Bowman, discloses adhesive compositionssuitable for use with fluorocarbon polymers comprising reactive(hydrolyzable) silanes or siloxanes. The fluorocarbon polymer isoptionally pretreated with a solution of sodium in ammonia or a solutionof sodium naphthalene in tetrahydrofuran.

U.S. Pat. No. 3,804,802, Bergna, discloses composites of thermoplasticresins, including polyfluoroolefins, with glass fiber materialsutilizing a nitrate-containing coupling agent to increase adhesionbetween the components.

U.S. Pat. Nos. 4,902,444 and 5,000,875 disclose melt processablefluorinated tetrafluoroethylene copolymer and terpolymer compositionsthat are treated with a fluorinating agent to remove unstable endgroups, then compounded with electrically conductive agents or thermallyconductive fillers.

U.S. Pat. No. 5,024,871, Arthur, discloses a ceramic-filledfluoropolymeric composite wherein the filler, preferably fused amorphoussilica, is coated with a zirconate or titanate coating. Said compositepossesses excellent thermal, mechanical and electrical properties. U.S.Pat. Nos. 5,061,548 and 5,384181, Arthur, disclose similar compositeswherein the surface coating agent is a silane.

SUMMARY OF THE INVENTION

This invention provides a process for the improvement of adhesionbetween fluoropolymers or fluorocopolymers and inorganic compoundscomprising the steps of 1) contacting the inorganic compound with anadhesion promoter to form a coated inorganic compound, 2) contacting thecoated inorganic compound with the fluoropolymer or fluorocopolymer and3) heating the coated inorganic compound-contacted, fluoropolymer orfluorocopolymer to a temperature sufficient to convert the adhesionpromoter to a carbon-rich char.

This invention also includes composite materials made by the process.These composites of an inorganic compound and a fluoropolymer orfluorocopolymer have an interfacial layer of carbon-rich char whichimproves the adhesion of the inorganic compound to the fluoropolymer orfluorocopolymer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic drawing of a sandwich specimen and the doublecantilevered beam (DCB) experimental setup.

FIG. 2 shows adhesion level (G) as a function of lamination temperaturefor Teflon® PFA 350 on Pyrex® glass.

FIG. 3 shows adhesion level (G) as a function of lamination temperaturefor Tefzel® 280 on Pyrex® glass.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention concerns a process for theimprovement of adhesion between fluoropolymers or fluorocopolymers andinorganic compounds.

Fluoropolymers or fluorocopolymers, hereinafter generally referred to asfluorochemicals, usable in the process of the present invention includepoly(tetrafluoroethylene), PTFE (Teflon®);tetrafluoroethylene-hexafluoro-propylene copolymer, also referred to asfluorinated ethylene-propylene copolymer, FEP (Teflon® FEP);ethylene-tetrafluoroethylene copolymer, ETFE (Tefzel®); perfluoroalkoxymodified tetrafluoroethylenes, PFA, e.g.,tetrafluoro-ethylene-perfluoropropylvinylether copolymer (Teflon® PFA);poly(chlorotri-fluorotetrafluoroethylene), PCTFE; vinylidenefluoride-tetrafluoroethylene copolymer, VF2-TFE; poly(vinylidenefluoride), (Kynar®); and tetrafluoro-ethylene-perfluorodioxolecopolymers, (Teflon AF®). Included in the group of fluoroelastomerswhich are fluoropolymers or fluorocopolymers are vinylidenefluoride-hexafluoropropylene copolymer (Viton® A); vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, (Viton® B);and tetrafluoroethylene-perfluoro(methylvinylether) plus cure sitemonomer terpolymer (Kalrez®).

Of the foregoing fluorochemicals preferred aretetrafluoroethylene-hexafluoropropylene copolymer,ethylene-tetrafluoroethylene copolymer, perfluoroalkoxy modifiedTetrafluoroethylenes and tetrafluoroethylene-perfluorodioxolecopolymers.

Inorganic compounds usable in the process of the present inventioninclude but are not limited to alumina, silica, titania, zirconia,silicate glass, aluminum nitride, silicon carbide and boron carbide. Theinorganic compound may exist in various physical forms, i.e., in theform of a powder, a solid shape having a surface, possibly a very smoothsurface, or in the shape of a surface coating on a secondary substrate.This secondary substrate could, for example, be a metal. One specificexample of the inorganic compound being a surface coating on a secondarysubstrate would be alumina on an aluminum metal secondary substrate.

Adhesion promoters usable in the process of the present invention areorganic compounds that convert to carbon-rich chars at elevatedtemperatures in non-oxidizing environments. Suitable compounds includebut are not limited to sucrose, sucrose octaacetate (SOA), abietic acid,poly(acrylic acid) and pitch.

Contacting the inorganic compound with the adhesion promoter may beaccomplished by various methods depending on the state of the inorganiccompound and the adhesion promoter. If, for example, the inorganiccompound comprises a solid surface, either as a monolithic structure oras a surface coating on a secondary substrate, the inorganic compound'ssurface may be coated with a solution of the adhesion promoter in acompatible solvent. Other methods of coating including but not limitedto deposition from a vapor phase, evaporation, sputtering, or evendusting the surface with powder are acceptable. If the inorganiccompound is in the form of a powder and the adhesion promoter is apowder, powder mixers, such as a Tekmar powder mill may be employed. Ifthe adhesion promoter is a liquid, the adhesion promoter may be added tothe inorganic compound, then mixed in, for example, a Spex mill. Eithersolid or liquid adhesion promoters may be dissolved in a compatiblesolvent and coated on a surface, as described above, or added to theinorganic compound powder in that form.

Optionally the adhesion-promoter contacted inorganic compound may bedried, if solvent wet, prior to contacting it further withfluorochemical.

Contacting the adhesion-promoted inorganic compound with a meltprocessable fluorochemical is accomplished in various means depending onthe physical form of both materials. If the adhesion-promoted inorganiccompound is in the form of a powder, this is accomplished most easily byadding the solid to the fluorochemical in its molten state and using,for example, a Haake® melt compounder to assure uniformity. It is alsopossible to mix solid polymer with solid adhesion-promoted inorganiccompound and heating the mixture prior to melting, accompanied by orprior to melting, followed by mixing. In either of the above twoembodiments, this contacting step and the subsequent heating to elevatedtemperatures may be conducted essentially simultaneously.

If the adhesion-promoted inorganic compound comprises a solid surface,either as a monolithic structure or as a surface coating on a secondarysubstrate, the adhesion-promoted inorganic compound surface may belaminated with a film of solid fluorochemical, coated with a suspensionof fluorochemical in an appropriate medium, or coated with a solution ofthe fluorochemical in a compatible solvent.

Heating the adhesion-promoted, inorganic compound-contactedfluorochemical to elevated temperatures to bring about the adhesive bondcan be carried out in a melt state as described above for powdered formadhesion-promoted inorganic compound or between heated rolls, insintering ovens or in a heated press for laminar structures.

The temperature utilized for bringing about the adhesive bond depends onthe nature of the adhesion promoter, the nature of the fluoropolymer offluorocopolymer and the nature of the inorganic compound. Thetemperature must be sufficient to convert the adhesion promoter to acarbon-rich char and below the temperature needed to thermally damagethe other materials present. Temperatures above 250° C. are usuallyemployed. Typically temperatures from 250° C. to about 375° C. areemployed.

Optionally, pressure may be applied during the heating stage. This ismost easily envisioned when the adhesion-promoted inorganic compound andfluorochemical are in laminar structure. If this general method is used,it is also optional to place pellets of the fluorochemical on thesurface of the adhesion promoted inorganic compound, and rely upon theapplied pressure to spread the fluorochemical onto the adhesion promotedsurface.

The utility of the products of the process of the present inventiondepend on the physical form of the fluorochemical-adhesion-promotedinorganic compound construct. If the adhesion-promoted inorganiccompound is in the form of a powder and this powder is dispersed in afluorochemical matrix, depending on the nature of the inorganiccompound, various advantageous properties may be developed in thefluorochemical-inorganic compound composite. For example, thermal orelectrical conductivity of said composite may be attractive. If thefluorochemical-inorganic compound construct comprises an inorganic oxidesurface-coated with a fluorochemical, the surface properties of saidinorganic oxide may be advantageously modified with respect to, forexample, durability, non-stick performance, corrosion resistance, orwetting characteristics.

Thermally conductive fluorochemicals are useful in heat exchanger tubesfor use at elevated temperatures in corrosive environments and fuserrolls in copier machines. For maximum utility, the material should beprocessable by extrusion, and the properties of the material in use mustinclude at least modest yield strength, failure strain and toughness.

In some cases, the filler material is selected to provide a high degreeof thermal conductivity. The most preferred inorganic filler material isaluminum oxide powder, selected for its moderate thermal conductivityand excellent chemical resistance. In order to increase the thermalconductivity of the composite by a factor greater than 2, it isnecessary to include greater than 20 volume percent of the highlythermally conductive phase into the fluorochemical. However, compositescontaining greater tan 60 volume percent filler are generallyunacceptable due to incomplete filling of the interparticle space withmatrix material. Even 20% by volume of filler has the potential ofcausing a problem in that the mechanical properties, such as ultimatestrength and strain to failure, are degraded substantially without largeincreases in yield stress. This results in a decrease in toughness ofthe composite and decreases the utility of the material. The mostaccepted reason for the degradation of the mechanical properties withincreased filler loading is that there is very little adhesion betweenthe fluorochemical matrix and the filler phase. Therefore, at a certainstrain, the interface fails, causing the formation of voids at thematrix/filler interface and subsequent yield and failure of the matrix.This invention relics on the creation of strong filler/matrix bonding sothat the potential problem of composites with low yield stress isavoided.

The problem of sub-standard mechanical properties in filledfluorochemicals, for example in Teflon® PFA 350, has been examinedpreviously. Teflon® PFA 350 is atetrafluoroethylene-perfluoropropylvinylether copolymer obtained fromthe DuPont Company, Wilmington, Del. Unfilled PFA 350 has a yield stressof 15.16 MPa (2200 psi), an ultimate strength of 27.58 MPa (4000 psi)and a strain to failure of 330%. At low loadings of aluminum oxide inPFA, the ultimate stress an the failure strain remain quite high, butdecrease as the aluminum oxide loading increases. At 5 and 10 percent byweight aluminum oxide, the ultimate stresses were 27.23 MPa (3950 psi)and 26.20 MPa (3800 psi), the yield stresses were 15.03 MPa (2180 psi)and 14.31 MPa (2075 psi), and the failure strains were 315% and 320%respectively. However, the thermal conductivity of these samples is onlymarginally improved over the unfilled matrix material. For moresubstantial improvements in thermal conductivity, upwards of 35% byweight aluminum oxide must be included in the fluorochemical matrix.

The mechanical properties of Teflon® PFA 350/aluminum oxide composites,especially where the filler loading is greater than 35% by weight butless than about 75% by weight, are improved by coating or mixing thealuminum oxide filler with a surface treating agent and subsequentlycombining the treated powder with the fluorochemical at an elevatedtemperature. The improved properties are believed to result fromimproved bonding of the filler powder to the fluoropolymer matrixrelative to similar materials that contain uncoated aluminum oxide. Thisimproved bonding results from the presence of an interfacial layer ofcarbon or carbon-rich char between the oxide and the fluoropolymer.

The most preferred filler used in the process of the present inventionis aluminum oxide, however, other inorganic powders with suitablethermal conductivities may be substituted. Preferred thermallyconductive fillers are defined as those with thermal conductivitiesgreater than 10 watts per meter degree Kelvin. Preferred thermallyconductive fillers are characterized by their having a small maximumprimary particle size combined with a low level of agglomeration. Somegrades of aluminum oxide usable in the process of the present inventioninclude Showa AL-45H (most preferred), Showa AS40, Showa AS-10, AlcoaA-16 and General Abrasives Type 55. Aluminum oxide of the gradesindicated can be purchased from Showa Denko America, New York, N.Y. andGeneral Abrasives Co., Niagara Falls, N.Y.

A wide variety of particle sizes can be used for the filler material.For use in melt compounding, the preferred size range will have a meanparticle size below 30 microns and a maximum less than 50 microns, withless than 10% of the material smaller than 0.1 microns. The mostpreferred distribution will have a mean less than 10 microns and amaximum less than 15 microns with less than 10% of the material smallerthan 0.1 microns. Filler powders substantially greater in size thanthose in this range result in composites that fail at very low strains,while composites made from fillers that are substantially smaller thanthe preferred range result in materials that arc difficult to processdue to their high viscosity during melt compounding. If processingroutes other than melt compounding are used, powders with averageparticle sizes less than 1 micron, with a substantial fraction below 0.1micron become more suitable.

Particle size distribution are conveniently determined with a HoribaLA-500 Laser Diffraction Particle Size Distribution Analyzer, availablefrom Horiba Instruments Incorporated, Irvine, Calif. Samples areprepared by adding 0.5 g of powder to a 50 ml pyrex beaker. To this isadded 20 ml of isopropanol. This mixture is sonicated until homogeneousin a Heat Systems Ultrasonics W-375 Sonicator/Cell Disruptor equippedwith a 0.5 inch diameter ultrasonic horn. The Horiba small samplediffraction cell is filled to two thirds of its capacity with isopropylalcohol, and the sonicated concentrate is added dropwise to thediffraction cell until the dilution is considered favorable asdetermined by the Horiba LA-500 instrument.

There are two important features of the surface treated filler particlesthat are adjusted so that the surface treated filler material canfunction in the process of the present invention. Small unagglomeratedparticles are needed in order to facilitate large strains-to-failure.Carbon coating of the filler particles causes adhesion between theparticles and the fluorochemical matrix and therefore producesreinforcement of the matrix. Both of these features are essential to theformation of composites with optimal mechanical properties. Small,unagglomerated particles that do not adhere to the matrix provide sitesfor nucleation of porosity at the particles surfaces, called cavitation,which leads to low yield stresses and necking of the composites. Largeor agglomerated particles that have good adhesion to the matrix causereinforcement at very small strains, however, they fail at low strainsrelative to those desired. Having both features present, smallunagglomerated particles and good particle/matrix adhesion, enables thefabrication of composites with the desired yield stress, ultimate stressand failure strain.

In the case of composites of this invention which are laminatedstructures improvement was measured in the adhesion of fluorochemicalsto inorganic compounds using a double-cantilever-beam (DCB) geometry.This method was employed, in constant displacement mode, withappropriate sandwich specimens produced by laminating a specifiedfluorochemical film between inorganic oxide beams at elevatedtemperature and pressure. The DCB method, when used with transparentbeams, has the advantage of allowing in situ observation of crack growthin a highly stabilizing crack-driving field. Adhesion is quantified inthis geometry from the elastic energy released, G, in the inorganicoxide beam on extension of the delamination at the interface between theinorganic oxide beam and the fluoropolymer or fluorocopolymer film.Adhesion in these systems depends strongly on lamination temperature,passing through a maximum.

Double cantilever beam specimens consisting of a thin layer offluorochemical inorganic oxide beams, as shown schematically in FIG. 1,were prepared using the following lamination procedure. The beam thatwere used were clean, highly-polished, inorganic oxide plates (1). Theplates were coated prior to lamination with an adhesion promoting agent-sucrose octaacetate, by dip coating the inorganic oxide substrate in awarm 2% solution of sucrose octaacetate in isopropanol. A piece offluorochemical film (2), 25 microns (0.001 S) thick, was carefullyplaced between two previously coated beams. The film was positioned toleave a section of the sandwich without film in order to generate astarter crack for mechanical testing. The loose sandwich was placedbetween Kapton®-lined tool steel platens in a hydraulic press and aforce of 9 kN (2000 lb.) gradually applied. The platens weresubsequently heated to a specified maximum temperature, at approximately500° C./hr, held at that maximum temperature for 10 minutes, and allowedto cool naturally under a power off condition. The residual pressure wasreleased and the Kapton® films stripped off the platens and theinorganic oxide beams. Excess fluorochemical film that was forced out ofthe sandwich during pressing was removed with a sharp blade.

Double-cantilever-beam (DCB) measurements were carried out in constantdisplacement mode by driving a steel blade (3) of known dimensions intothe region without the fluorochemical film. The sandwich and bladearrangement were set up on a universal servo-hydraulic testing machine(Model 8100, MTS Systems Corp, Eden Prairie, Minn.) and the delaminationbetween the inorganic beams and the fluorochemical observed intransmitted light using a long (40 cm) working distance microscope withan encoded vertical-travel stage, accurate to 10 microns (Questar QM100, Questar Corp, New Hope, Pa.). The testing procedure consists ofdriving the steel blade into the interface and observing the subsequentdelamination motion with the microscope. Because many inorganic oxidesand thin fluoropolymer and fluorocopolymer films are transparent, theresulting crack could be imaged in transmitted light. The crack tip isclearly delineated by Fizeau interference fringes as crack openingdisplacements close to the tip approach dimensions of the wavelength oflight. Delamination size measurements were made as a function of time,and the threshold crack length identified as the condition when no crackextension was observed over a 24 hr period.

Adhesion, in terms of the energy release rate, G, was determined for aconstant wedging displacement, by noting that the energy required toseparate the interface on unit crack extension is equal to the elasticstrain energy released in the two cantilever beams as they unload oncrack propagation: ##EQU1## with d the beam thickness; E, the planestrain Youngs's modulus; c, the crack length(4) (As in B. R. Lawn,"Fracture of Brittle Solids", Cambridge University Press, 1993). Bladethickness (2h) and beam thickness were measured to 1 mm accuracy using amicrometer prior to specimen fabrication.

FIG. 2 plots the measured threshold adhesion level, G, as a function ofmaximum lamination temperature for the Teflon® PFA 350/sucroseacetate/Pyrex® system. Several features may be noted from these data.First, the adhesion level is strongly dependent on laminationtemperature, starting at some 10 J.m⁻² at 315° C. and increasing to amaximum of some 60 J.m⁻² at around 345° C. Second, the adhesion appearsto peak before decreasing at still higher temperatures. Excessivetemperatures at which polymer degradation occurs may determine the upperlamination temperatures and lead to a peak in the adhesion energy.

FIG. 3 plots the measured threshold adhesion level, G, as a function ofmaximum lamination temperature for the Tefzel® 280/sucroseacetate/Pyrex® system. Again the adhesion levels are strongly dependenton lamination temperatures, passing through a maximum.

The Tefzel® 280 containing laminate demonstrates higher adhesion levelsthan the Teflon® PFA 350 containing laminate and that peak adhesion isobtained at a lower processing temperature.

EXAMPLES COMPARATIVE EXAMPLE 1 Pyrex®/SOA/PFA 350 Laminates

Sucrose octaacetate (2 parts) was dissolved in warm isopropanol (98parts). Pyrex® plates, 25.4 mm×76.2 mm×0.762 mm(1S×3S×0.030), that hadbeen polished on their largest faces, and degreased in isopropanol, weredipped into the warm sucrose octaacetate solution, leaving approximatelyone centimeter of the bar protruding above the surface of the solutionfor the purpose of gripping. The bars were withdrawn from the solutionat a rate of roughly 15 centimeters per second, to yield a thin layer ofsolution adhering to the bar. The coated bar was allowed to dry underambient conditions, leaving a white translucent layer of sucroseoctaacetate on the surfaces of the bars. A film of Teflon® PFA 350, 25microns thick, was placed over the broad side of one of the bars,leaving roughly 2 centimeters of the bar exposed at one end. On top ofthe Teflon® PFA 350 film was placed a second coated Pyrex® bar. Theloose sandwich was placed between the Kapton®-lined tool steel platensof hydraulic press and a force of 9 kN (2000 lb.) gradually applied. Theplatens were subsequently heated to 315 degrees centigrade, atapproximately 500° C./hr, held at that maximum temperature for 10minutes, and allowed to cool naturally under a power off condition(overnight). The residual pressure was released and the Kapton® wasmechanically stripped from the surfaces of the beams. The resultingdouble cantilever beams were tested as previously described. Adhesionresults are shown in Table 1. Additional samples prepared in this methodwere immersed in boiling water and were shown to resist delamination formore than 24 hours.

COMPARATIVE EXAMPLE 2 AND INVENTIVE EXAMPLES 3-4

These examples are identical to Example 1 with the exception that themaximum heating temperatures during pressing was varied. The conditionsand results are shown in Table 1.

INVENTIVE EXAMPLES 5-7

These examples are identical to Example 1 , with the exception that thefluorocopolymers used between the two Pyrex® plates was Tefzel® 280 witha thickness of 100 microns before pressing. The processing conditionsand results are shown in Table 1.

COMPARATIVE EXAMPLE 8

This sample is identical to Example 1 with the exceptions that thePyrex® bars were not coated with an adhesion promoting material prior tothe lamination process, and the peak temperature during pressing was 350degrees centigrade. Additional samples prepared in this method wereimmersed in boiling water and were shown to delaminate in less than 1hour.

COMPARATIVE EXAMPLE 9

This sample is identical to example 5 with the exceptions that thePyrex® bars are not coated with an adhesion promoting material prior tothe lamination process, and the peak temperature during pressing was 295degrees centigrade.

                  TABLE 1    ______________________________________    Processing conditions and results for Examples 1 through 7    and Comparative Examples 8 and 9    Example     Maximum      Energy Release    Number      Temperature (° C.)                             Rate (J/m.sup.2)    ______________________________________    1           315          9    2           335          16    3           345          62    4           355          48    5           295          42    6           320          188    7           332          155    8           350          15    9           295          15    ______________________________________

COMPARATIVE EXAMPLE 10 Composite with untreated Showa aluminum oxide asfiller material.

65 parts Teflon® PFA 350 was melted in a 70 cubic centimeter Haake(mixer equipped with roller blades at 350° C. while running at 15 RPM.When the polymer was molten, 35 parts aluminum oxide (Showa AL-45H) wasslowly added to the polymer. After the addition of the inorganic powder,the speed of the mixer was increased to 100 RPM, where it was heldconstant for 30 minutes. The temperature of the mixture was maintainedat 350° C. by air cooling the mixer. After mixing, the cover of themixer was removed, and the compounded polymer was cut out of theapparatus using copper/beryllium alloy knives. The irregularly shapedchunks were placed between steel plates lined with Kapton® film, andpressed to a thickness of roughly 2.5 mm (0.1 inches) using a CarverPress with heated platens that had been preheated to 350° C. Kapton® isa thermally stable polyimide film obtainable from the DuPont Company,Wilmington, Del. The steel/Kapton®/compounded PFA assembly was thenmoved to a water cooled press and cooled to room temperature while underpressure. The thick sheet was cut into smaller pieces, half of whichwere reloaded between Kapton® lined steel plates and reheated in theCarver press. The material was pressed to a thickness of roughly 1 mm(0.04 inches). The steel/Kapton®/compounded PFA assembly was then cooledas before in a water cooled press. Tensile bars (ASTM D1708) were cutfrom the sheets and tested in an Instron load frame that was equippedwith a 91 kg (200 pound) load cell at 0.21 mm/sec (0.5 inches perminute). The results are reported in Table 2. Some of this material wasobserved while under strain in an optical microscope at a magnificationof 200×. Suitable Leitz microscopes are available from Leica, Inc.,Malvern, Pa. Cavitation at the matrix/filler interface was visible atstrains less than 50%, and persisted until failure of the material.

COMPARATIVE EXAMPLE 11 Composite with calcined General Abrasive Type 55aluminum oxide as filler material

35 parts aluminum oxide (General Abrasives Type 55) was calcined at 800°C. in air for two hours, and allowed to cool to room temperature. Thepreviously heated powder was then added to the polymer in place of theShowa AL-45H and processed as in Comparative Example 10.

INVENTIVE EXAMPLE 12 Composite with poly(acrylic acid) treated Showaaluminum oxide as filler material

50 parts aluminum oxide (Showa AL-45H) was loaded into a Tekmar PowderMill. One part of poly(acrylic acid) powder, molecular weight 5000 waspoured over the top of the aluminum oxide. Poly(acrylic acid) can beobtained from Aldrich Chemical Company, Milwaukee, Wisconsin, The coverwas installed on the mill and the mill started. The resulting powdermixture appeared to be uniform in color. 35 parts of this powder mixturewas added to molten Teflon® PFA 350 as in example 10, substituted forthe Showa AL-45H aluminum oxide.

INVENTIVE EXAMPLE 13 Composite with abietic acid treated Showa aluminumoxide as filler material

50 parts aluminum oxide (Showa AL-45H) was loaded into a Spex VibratoryMill. One part of abietic acid powder was poured over the top of thealuminum oxide powder. Abietic acid is obtainable from the FlukaChemical Company, New York, N.Y. The cover was installed on the mill andthe mill started. The resulting powder mixture appeared to be uniform incolor. 35 parts of this powder mixture was added to molten Teflon® PFA350 as in Example 10, substituted for the Showa AL-45H aluminum oxide.

INVENTIVE EXAMPLE 14 Composite with sucrose octaacetate treated Showaaluminum oxide as filler material

50 parts aluminum oxide (Showa AL-45H) was loaded into a Spex VibratoryMill. One part of sucrose octaacetate powder was poured over the top ofthe aluminum oxide powder. Sucrose octaacetate can be obtained fromAldrich Chemical Company, Milwaukee, Wis. The cover was installed on themill and the mill started. The resulting powder appeared to be uniformin color. 35 parts of this powder mixture was added to molten Teflon®PFA as in Example 10, substituted for the Showa AL-45H aluminum oxide.Some of this material were observed while under strain in an opticalmicroscope at a magnification of 200×. Cavitation at the matrix/fillerinterface was not observed up until and including the failure of thematerial.

INVENTIVE EXAMPLE 15 Composite with calcined sucrose octaacetate treatedShowa aluminum oxide as filler material

This example was identical to Example 14 with the exception that themixed aluminum oxide/sucrose octaacetate powders were calcined undernitrogen at 375° C. for one hour and allowed to cool to room temperatureunder nitrogen prior to mixing the powders with the Teflon® PFA 350.

                  TABLE 2    ______________________________________    Mechanical Properties of Example 10 to 15              MAXIMUM STRESS    EXAMPLE NO.              (MPa/PSI)     STRAIN AT FAILURE (%)    ______________________________________    10        15.38/2231     211*    11        18.31/2656     268*    12        19.30/2799    102    13        22.27/3230    123    14        22.50/3263    159    15        19.93/2891    155    ______________________________________     *Indicates that the sample formed a stable neck that extended until it     reached the tabs on the tensile bar, at which point the sample broke

ADDITIONAL EXAMPLES

In Examples 16 through 26, all compositions are described in parts byweight. Unless otherwise indicated in Table 3, sucrose octaacetate isused as the material to promote adhesion between the filler particlesand the fluoropolymer matrix. Where present, the amount of sucroseoctaacetate is indexed to the amount of aluminum oxide at 1 part ofsucrose octaacetate to 10 parts aluminum oxide. The thermal conductivityis compiled in W/m/0K, while the failure strain is listed in percent andthe yield stress is compiled in megapascals (pounds per square inch).All of these samples were mixed as in Example 14 above. In Table 3, allcomposites except Example 21 use Showa's AL-45H aluminum oxide powder asthe filler. Example 21 uses Showa's AS-10 aluminum oxide powder as thefiller Examples 16, 22, 24 and 25 were fabricated without sucroseoctaacetate as comparative examples, while Samples 23 and 24 use PFA 450as the fluoropolymer matrix and Examples 26 and 27 use FEP 140 as thefluoropolymer matrix.

    ______________________________________          Parts                  Failure          Fluoro- Parts    Failure                                 Stress    Thermal    EX. # polymer Alumina  Strain                                 (MPa/psi) Conductivity    ______________________________________    16    100     0***     337   25.70/3728                                           .22    17    65      35       124   22.40/3249                                           .44    18    55      45       97    26.55/3851                                           .68    19    45      55       58    25.86/3751                                           .92    20    35      65       36    25.85/3749                                           1.14    21    35      65*                      1.36    22    65**    35***    256   15.62/2251****    23    65**    35       148   24.99/3625    24    65      35***    254   18.73/2716****    25    65@     35***    62    17.40/2523    26    65@     35       120   2105/3053    ______________________________________     *Showa AS10     **Post Fluorinated PFA 350 as matrix (PFA 450)     ***No sucrose octaacetate (Comparative Examples)     ****sample necked     @ FEP 140 as matrix

We claim:
 1. A process for improving adhesion between an inorganiccompound and a fluorochemical selected from the group consisting offluorohomopolymers, fluorocopolymer and fluoroelastomers comprisingcontacting the inorganic compound with an adhesion promoter to form acoated inorganic compound, contacting the coated inorganic compound withthe fluorochemical to form a fluorochemical-coated inorganic compoundcombination and heating the combination to a temperature sufficient toconvert the adhesion promoter to a carbon-rich char.
 2. The process ofclaim 1 wherein the adhesion promoter is a member selected from thegroup consisting of sucrose, sucrose octaacetate, abietic acid,poly(acrylic acid), and pitch.
 3. The process of claim 1 wherein theinorganic compound is a member selected from the group consisting ofalumina, silica, titania, zirconia, silicate glass, aluminum nitride,silicon carbide and boron carbide.
 4. The process of claim 2 wherein theinorganic compound is a member selected from the group consisting ofalumina, silica, titania, zirconia, silicate glass, aluminum nitride,silicon carbide and boron carbide.
 5. The process of claim 1 wherein thefluorochemical is a member selected from the group consisting ofpoly(tetrafluoroethylene), tetrafluoroethylene-hexafluoropropylenecopolymer, ethylene-tetrafluoroethylene copolymer, perfluoroalkoxymodified tetrafluoroethylenes, poly(chlorotrifluorotetrafluoroethylene),vinylidene fluoride-tetrafluoroethylene copolymer, poly(vinylidenefluoride), tetrafluoroethylene-perfluorodioxole copolymers, vinylidenefluoride-hexafluoropropylene copolymer, vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, andtetrafluoroethylene-perfluoromethylvinylether) plus cure site monomerterpolymer.
 6. The process of claim 4 wherein the fluorochemical is amember selected from the group consisting of poly(tetrafluoroethylene),tetrafluoroethylene-hexafluoropropylene copolymer,ethylene-tetrafluoroethylene copolymer, perfluoroalkoxy modifiedtetrafluoroethylenes, poly(chlorotrifluorotetrafluoroethylene),vinylidene fluoride-tetrafluoroethylene copolymer, poly(vinylidenefluoride), tetrafluoroethylene-perfluorodioxole copolymers, vinylidenefluoride-hexafluoropropylene copolymer, vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, andtetrafluoroethylene-perfluoromethylvinylether) plus cure site monomerterpolymer.
 7. The process of claim 6 wherein the fluorochemical is amember selected from the group consisting ofethylene-tetrafluoroethylene copolymer,tetrafluoroethylene-hexafluoropropylene copolymer, perfluoroalkoxymodified tetrafluoroethylenes, and tetrafluoroethylene-perfluorodioxolecopolymers.
 8. The process of claim 1 wherein the combination is heatedto a temperature of from about 250° C. to about 375° C.
 9. The processof claim 4 wherein the combination is heated to a temperature of fromabout 250° C. to about 375° C.
 10. The process of claim 5 wherein thecombination is heated to a temperature of from about 250° C. to about375° C.
 11. The process of claim 7 wherein the combination is heated toa temperature of from about 250° C. to about 375° C.
 12. The process ofclaim 11 wherein the inorganic compound is alumina.
 13. A composite ofan inorganic compound and a fluorochemical selected from the groupconsisting of fluorohomopolymers, fluorocopolymers and fluoroelastomerswherein the inorganic compound is bonded to the fluorochemical by aninterfacial layer of carbon-rich char.
 14. The composite of claim 13wherein the inorganic compound is a member selected from the groupconsisting of alumina, silica, titania, zirconia, silicate glass,aluminum nitride, silicon carbide and boron carbide.
 15. The compositeof claim 13 wherein the fluorochemical is a member selected from thegroup consisting of poly(tetrafluoroethylene),tetrafluoroethylene-hexafluoropropylene copolymer,ethylene-tetrafluoroethylene copolymer, perfluoroalkoxy modifiedtetrafluoroethylenes, poly(chlorotrifluorotetrafluoroethylene),vinylidene fluoride-tetrafluoroethylene copolymer, poly(vinylidenefluoride), tetrafluoroethylene-perfluorodioxole copolymers, vinylidenefluoride-hexafluoropropylene copolymer, vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, andtetrafluoroethylene-perfluoromethylvinylether) plus cure site monomerterpolymer.
 16. The composite of claim 13 wherein the fluorochemical isa member selected from the group consisting ofethylene-tetrafluoroethylene copolymer,tetrafluoroethylene-hexafluoropropylene copolymer, perfluoroalkoxymodified tetrafluoroethylenes, and tetrafluoroethylene-perfluorodioxolecopolymers.
 17. The composite of claim 14 wherein the fluorochemical isa member selected from the group consistingofethylene-tetrafluoroethylene copolymer,tetrafluoroethylene-hexafluoropropylene copolymer, perfluoroalkoxymodified tetrafluoroethylenes, and tetrafluoroethylene-perfluorodioxolecopolymers.
 18. The composite of claim 17 wherein the inorganic compoundis alumina.
 19. The composite of claim 17 wherein the composite is fromabout 35% by weight to about 75% by weight alumina.
 20. A composite ofclaim 14 wherein the inorganic compound is in the form of solid surface.21. A composite of claim 14 wherein the inorganic compound is in theform of a powder.
 22. A composite of claim 18 wherein the inorganiccompound is in the form of a solid surface.
 23. A composite of claim 18wherein the inorganic compound is in the form of a powder.